This paper reports a laboratory study of the gas breakthrough pressure for different gas/liquid systems in the Mississippian-age Midale Evaporite. This low-permeability rock formation is the seal rock for the Weyburn Field in southeastern Saskatchewan, Canada, where CO 2 is being injected into an oil reservoir for enhanced recovery and CO 2 storage. A technique for experimentally determining CO 2 breakthrough pressure at reservoir conditions is presented. Breakthrough pressures for N 2 , CO 2 and CH 4 were measured with the selected seal-rock samples. The maximum breakthrough pressure is over 30 MPa for N 2 and approximately 21 MPa for CO 2 . The experimental results demonstrate that the Weyburn Midale Evaporite seal rock is of high sealing quality. Therefore, the Weyburn reservoir and Midale Beds can be used as a CO 2 storage site after abandonment. The measured results also show that the breakthrough pressure of a seal rock for a gas is nearly proportional to the interfacial tension of the gas/brine system. The breakthrough pressure of a CO 2 /brine system is significantly reduced compared with that of a CH 4 /brine system because of the much lower interfacial tension of the former. This implies that a seal rock that seals the original gas in a gas reservoir or an oil reservoir with a gas cap may not be tight enough to seal the injected CO 2 if the pressure during or after CO 2 injection is the same or higher than the original reservoir pressure. Therefore, reevaluation of the breakthrough pressure of seal rocks for a given reservoir is necessary and of highest priority once it is chosen as a CO 2 storage site.

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